Such an apparatus is opened or disengaged and closed or engaged by powering one of two control coils associated with the apparatus. Traditionally, each coil is powered by a low voltage electrical signal produced from the relay building by closing an order-transmitting contact, e.g., a local control pushbutton or a contact of a remote protection relay.
On/off orders are conveyed over a low voltage link constituted mainly by a bundle of conductive cables forming as many electrical lines extending between the building and the apparatus, with each line serving to convey a single on/off order in the form of a voltage step of about 250 volts. Such a link is relatively long, about 100 meters to 200 meters. In addition, signals indicating the state of the electrical apparatus (in particular whether it is in the engaged state or the disengaged state) as delivered by auxiliary contacts of the kind described in French patent application No. 92/06920 are returned via these electrical lines to enable the operator to make decisions concerning engagement and disengagement.
Auxiliary contacts are now designed around electronic circuits as described in French patent application No. 94/11638, and coil control relays have been replaced by static switches based on transistors. Under such conditions, it is necessary to isolate the electronic circuits from the high voltages in common mode that can appear on the electrical lines used for transmitting on/off orders. To avoid any risk of the electronic circuits being destroyed, consideration has therefore been given to isolating the electronic circuit of the electrical lines proper by a system comprising two stages of electrical/optical conversion. More particularly, as shown in FIG. 1, at the end of each electrical line such as the line 1, there is connected in series an electro-optical conversion unit 11 which thus converts a voltage step into an optical signal having two logic states, and an opto-electrical conversion unit 12 connected to the unit 11 via an optical fiber 10 which serves to convert the optical signal output by the unit 11 into a two-state electrical signal suitable for use as a control signal by a processor such as 13, the processor being connected to auxiliary contacts such as 14 and to static switches for controlling the coils such as 15 of the apparatus 16. These orders, in particular commands, must be transmitted to the apparatus with as little delay as possible.
In such a setup, a problem of crosstalk has been observed between adjacent electrical lines in the link, which problem can give rise to general malfunction of the control of the electrical apparatus.
More particularly, with reference to FIG. 1, when contact C2 close to the relay building BR is closed, a voltage step S2 is applied to the line 2, and this electrical signal is received by the associated electro-optical conversion unit (not shown) to be converted into an electrical control signal for the processor 13.
However, because the link cables are not shielded from one another and because the link is relatively long, transmission of voltage step S2 of level V on line 2 causes a voltage peak S1 of level V to be conveyed on adjacent line 1 even though contact C1 had remained open. This voltage peak S1 tapers off (as shown in the figure) in application of a relationship that depends on the structure of the unit 11 connected to the end of the line 1 (relationship may be exponential, for example, if the unit 11 can be approximated as a resistance). The peak is detected by the unit 11 and then converted into an optical signal for the unit 12, with this relatively short optical signal nevertheless being detected by the unit 12 because of the intrinsic speed and sensitivity thereof, and finally being converted into an electrical control signal for the processor 13. This means that a wrong order is applied to the processor 13 and that can be detrimental to proper operation of the system.